PITTSBURGH (KDKA) — Prosecutors have responded to a defense lawyer’s claim that a former Plum High School teacher should not be prosecuted for a sex crime because the girl was 18 and a consenting adult.

Jason Cooper is headed to trial on charges of institutional sexual assault based in part on a female student’s description of their relationship during a preliminary hearing.

But his lawyer argues that because the girl was 18, there was no crime.

The defense filed motions arguing that charge should be dismissed because it punishes the right of consenting adults to engage in private sexual conduct without governmental interference.

“Once an individual turns 18 there is no legislation that exists to restrict the actions of an adult, who and what they can do behind closed doors,” said Mike Deriso, the defense attorney.

“It does not become open season on a student just because that student turns 18,” Allegheny County District Attorney Stephen Zappala said.

The prosecution disagreed on the age issue, but also filed a motion this week.

It argues that the relationship between Cooper and the girl did not begin on Jan. 6, which was her 18th birthday, and while it’s alleged their sexual encounters began after that, it was nurtured through the previous fall semester and Christmas break during and after school hours when, in the words of the prosecutor, the defendant began to groom her leading up to the sexual encounter.

The prosecution argues that what it calls the grooming of the girl was just as immoral as the sexual relationship that occurred after she turned 18.

Grooming, say prosecutors, is a necessary step in order to lay the foundation for the sexual relationship.

The judge presiding over this case is scheduled to hear pre-trial motions next month.